You left out OK and O.K.! Why are there so many ways to spell this, honestly. In proper writerly stuff, I go with 'okay', but casually I use 'ok' all the time.
I use okay in writing but in dialogue and real life I use ok. But really, they all sound the same so there's only some distinction.
I read somewhere that 'OK' is the acceptable way to do it, but I've never been fond of it, so I always use 'okay.' I hope it doesn't turn out to be a deal-breaker with some publisher or other.
I *think* I've only seen 'okay' in modern trad published books, but I can't be certain. I use okay and my agent didn't change it in her revisions.
I have honestly never seen it written down as O.k (or some variant thereof with the dot in betwixt) I always wrote it down as Okay, though I have seen it written as Ok.
It's obviously because I'm so used to 'okay' but 'ok' looks truncated to me. If I see it I want to get a red pencil and add '-ay' to the end.
If you're gonna write it as OK, both letters should be capitalized (if you're writing standard English at any rate).
I know that it STARTED as just two letters (OK), but there's something about my brain that refuses to accept that as a full word in its own right. I use "OK" if I'm texting someone, but in every other situation I prefer "okay".
I started with OK, but went to "okay," because I could do more with it. Like have a character respond, "Okaaaaayyy?" dubiously.